Offshore oil and gas production platforms include fixed platforms supported above the sea surface by fixed legs, dug into the sea floor. These platforms are one mechanism for harvesting oil and gas from wells which have been drilled into fields located beneath the sea floor. Much as with conventional on-shore drilling, these platforms use a derrick and associated equipment to perform the actual drilling operation prior to oil or gas production. Once oil or gas is struck, capping equipment is used to contain the well and to govern removal of the oil for storage,, transportation and refinement. The drill floor package (typically including the derrick, drill floor and substructure) is then no longer needed and consequently, for all but the largest platforms, is removed and used to drill another nearby well or is removed to some other remote location for drilling. In this manner, the same drill floor package can be advantageously used on numerous fixed platforms. When the drill floor package is removed, the fixed platform becomes merely a production platform, no longer having drilling capabilities.
An offshore platform typically includes anywhere from four to forty drilling positions that may be used to drill wells into at least one production field below the sea floor. Consequently, the offshore platform serves as a central collection point for oil and gas obtained from the wells, which may be extend downwardly and outwardly in many directions through the sea floor. The larger the size of the production field, the larger the size of fixed platform used to collect the oil or gas taken from the field.
Typically, an offshore platform is positioned above a promising field in a manner that allows the most efficient drilling of this multiplicity of wells. Thus, the drill floor package and other drilling equipment are generally used over a short span of time to drill a number of proximate wells at a time when the platform is first constructed. However, production requirements and changes in capacity may require the drilling of additional wells at times after the original drilling process has been completed. It is therefore often desirable for the drill floor package to be brought back to the fixed platform, so that additional wells may be drilled to increase the production of oil harvested by the fixed platform, or for other reasons. It is, for example, sometimes desirable that the drill floor package be replaced on the fixed, production platform so that existing wells may be "reworked" to maintain a desired level of production.
As an alternate method to assembling the drill floor package (which may weigh as much as three-million pounds and extend one-hundred and fifty feet into the air) directly upon the upper surface of the platform, a floatable vessel, called a "jack-up rig" may also be used to drill the wells for the fixed platform. The jack-up rig, after completion of the drilling operation, is towed to other locations to provide other drilling services.
The jack-up rig is essentially a mobile drilling facility having everything necessary to support drilling operations, including crew facilities, storage tanks for fluid supply and storage, a derrick, and some drilling support equipment, such as well control equipment and the like. Roughly shaped like the home plate of a baseball diamond, the jack-up rig mounts three downwardly extendable legs which it extends into the sea floor in order to lift its hull above the surface of the water to perform the actual drilling, significantly insulated from the effect of wind and waves. When the rig is floating, the downwardly extendable legs may be moved with respect to the rig. When the legs are resting upon the sea floor, the hull may be moved with respect to the legs, above the surface of the water. Jack-up rigs are generally either used to perform exploration drilling or to perform production drilling over a fixed platform.
The design of early jack-up rigs evolved into what is commonly known as a "slot-type" jack-up rig. These rigs feature a derrick that movably overlies a slot existing in the aft end of the jack-up rig. Typically, the slot is sufficiently large that the jack-up rig may be positioned about a small fixed platform, which is entirely engulfed within the slot. The hull of the jack-up rig is then elevated so that the hull of the rig is raised above the fixed platform, and the derrick is moved over the slot to drill a limited number of wells through the slot and the fixed platform.
In recent decades, however, the tendency has been for fixed, offshore platforms to grow in size. Primarily, these larger platforms are used for production drilling of larger fields in relatively deeper waters, and are needed to withstand the more extreme weather and wave conditions that exist in that environment. In addition, the larger platforms are also able to sustain a larger number of well positions, corresponding to larger field size. With these large platforms, drilling equipment, including the drill floor package, may be permanently stationed upon the fixed platform.
The permanent installation of drill floor packages aboard platforms has certain problems, however. First, the drill floor package, which is a large and expensive piece of equipment, is used only for a short period and remains idle, when it could be used elsewhere. Second, the permanent installation of drill floor packages requires extensive support facilities, storage tanks, crew quarters, and the like. This requires much space aboard the fixed platform and requires much expense incurred only for the relatively-short duration drilling procedures. Third, in more recent years, the larger production fields are harder to find, and thus, the recent trend has been for somewhat smaller platforms to be constructed for production from a smaller number of wells. Thus, permanent installation of the drill floor package aboard a fixed, offshore platform is tending to become less economical.
Since many of the present day platforms are too large to accommodate drilling in the slot-mode (the platforms are too large to fit within the jack-up rig's slot), many jack-up rigs have been constructed to operate in a "cantilever mode." These jack-up rigs do not have a slot defined by their aft ends, but rather, have a cantilever structure that may be extended over the aft end of the jack-up rig and retracted to a stowed position aboard the jack-up rig. The drill floor package is typically mounted at the aft end of the cantilever structure. Thus, when it is desired to drill an oil or gas well from above a fixed platform, the jack-up rig is maneuvered adjacent to the fixed platform and its hull elevated above the sea surface and above the fixed platform. The cantilever is then extended over the desired drill slot and drilling occurs above and through the fixed platform.
The cantilever-type jack-up rigs have become quite popular, especially since they may be used with the larger platforms. Operators of slot-type jack-up rigs have thereby faced an economic incentive to adapt their rigs to use with large platforms, and to thereby remain competitive with the cantilever-type rigs. In part to address this problem, a method of "tender assist" drilling has been developed wherein the drill floor structure is skidded across from the deck of the jack-up rig's hull onto the fixed platform. When the jack-up rig is positioned with its aft end adjacent to the fixed platform, the hull of the jack-up rig is elevated to exactly the level of the fixed platform. A "pony base" is then pushed onto the upper surface of the fixed platform, and is supported by "capping beams" of the fixed platform (generally two parallel I-beams) that are capable of supporting the pony base. The drill floor package serves as a balancing load during this process, supporting the pony base by a pinned connection. That is, the pony base is not supported by a cantilever structure, but is coupled to the drill floor package which thereby keeps the pony base from tilting during the transfer procedure. Once the pony base is supported upon the capping beams, it is disconnected from the drill floor assembly, the hull of the jack-up rig is elevated until its upper surface is on a horizontal level with the top of the pony base, and a bridge structure is erected between the hull and the pony base. The drill floor package is then pushed across the bridge and onto the top of the skid base, and drilling is performed with the drill floor package continually supported by the fixed platform. The jack-up rig, its crew facilities and support equipment support the actual drilling operations. This type of tender assist drilling is generally described in PCT publication number WO 92/08007.
However, operating a jack-up rig in the cantilever mode in deeper waters (as deep or deeper than three-hundred feet) can present several difficulties. First, extreme weather conditions will frequently cause relative motion between the jack-up rig and the platform which will cause drilling operations to be suspended. Second, it is difficult to maneuver the jack-up rig sufficiently close to the platform for the cantilever structure to reach sufficiently onto the platform in order that all desired drilling positions may be accessed.
In partial response, cantilever-type jack-up rig operators have also developed their own methods of tender assist drilling which also use a procedure by which the drill floor package is loaded onto a fixed platform. These methods also present the advantage that the drill floor package is supported entirely upon the fixed platform, enabling drilling to continue in relatively harsh weather conditions, and over a larger number of possible well positions.
One such method for tender assist drilling using a cantilever jack-up rig is generally described in U.S. Pat. Nos. 4,938,628 and 5,052,860 to Ingle. The drill floor package is positioned at the aft end of the cantilever structure, which is extended in overlapping bracketing relation with the capping beams. Since the spacing of the lateral cantilever beams of the cantilever structure is approximately sixty feet, and since capping beam spacings generally vary between forty and fifty-five feet, the cantilever beams are used to place the drill floor package directly above the fore ends of the capping beams. The hull of the jack-up rig is then lowered, such that the capping beams lift the drill floor package directly off the cantilever structure, and continue to support the drill floor package during drilling operations.
These methods work relatively well and facilitate continued drilling under harsh weather conditions, because any relative motion between jack-up and platform no longer affects the drilling operation. They are not, however, without disadvantages. In particular, offshore drilling platforms are built in many configurations and styles by different operators. Thus, drill floor packages must generally be specially adapted to the design of the drilling platform, such that the framework of the drill floor package structure is properly supported upon the platform's capping beams. Depending upon the configuration of offshore platform, capping beams generally vary in spacing between 40 feet to 55 feet, and there is no uniform standard of construction. Also, since most fixed platforms feature decking that is positioned about and between the capping beams, the latter-described method of tender assist drilling must overcome a significant obstacle in an endeavor to place the drill floor package over any desired drill slot on the fixed platform.
Significantly, the transfer of the drill floor package, known as a "skid-off" or "skidding" procedure, presents inherent safety concerns. Basically, these methods involve the transfer of a three-million pound structure between two separated platforms which are both elevated a significant distance above the water. Although drilling itself is facilitated in relatively harsh weather conditions, the skid-off transference procedure requires relatively calm conditions, and thus, ties up use and location of the jack-up rig in attendance of calm weather for placement or removal of the drill floor package. This may require the attendance of a jack-up rig for as many as three weeks in some environments (such as the Central North Sea), awaiting an appropriate weather window. Aside from the typical $50,000 per day rental costs that are lost by the jack-up operator (jack-up rig mobilizations are very often lump sum transactions), production drilling is also delayed.
These methods also have certain other limitations. For example, during the "skid-off" procedure, there is a period when the drill floor package rests both upon the capping beams of the fixed platform and the cantilever beams. This can impose undesired side loads which are detrimental to both the cantilever structure and the capping beams of the fixed platform, because waves and weather may cause relative motion between the jack-up rig and the fixed platform during the skid-off procedure. Also, the jack-up rig must be positioned very closely to the fixed platform, and very accurately aligned therewith, which requires the most benign weather conditions. Although sophisticated alignment methods enable generally accurate alignment between the jack-up rig and the fixed platforms, there may be some limited misalignment between the cantilever beams and the capping beams, which causes these undesired side loads to be imposed upon one or both during skidding. This effect, as mentioned, may significantly heightened during harsh weather conditions. When such misalignment and relative movement occurs, the longitudinal capping beams are subjected to non-intended, non-vertical loads that may threaten the integrity of the platform structure and reduce safety factors.
Therefore, mobilization of a jack-up rig for a skidding operation may entail a significant amount of unused time which is devoted solely to awaiting ideal weather conditions. In addition, removal of the drill floor package also presents difficulties and consumes time, as the legs of the jack-up rig may settle into the sea floor, creating or amplifying misalignment between the fixed platform and the rig and delaying removal while the problems are cured.
Because tender assist drilling is now being applied to deep water and harsh environments, it is even more arduous to accurately position the jack-up rig alongside the fixed platform, unless the weather is very calm. Current skid-off methods do not work if the misalignment exceeds quite low values. When combined with the problem of undesired side loads and differing construction standards for capping beam surfaces, it is apparent that current skid-off methods have some significant drawbacks.
Thus, a need exists for a flexible approach to the skid-off process that may be safely performed in relatively harsh weather conditions, under circumstances in which there is relative motion between the jack-up rig and the fixed platform, and in which the jack-up rig need only be positioned within a tolerance not acceptable to current practice. Furthermore, a need exists for a practical method of transferring a drill floor package to and from offshore platforms and that can provide drilling access to all of the drill slot locations on the larger platforms. A need also exists for a drill floor package support structure that can be adapted on-site to be loaded upon the upper platform surfaces of nearly any offshore platform, irrespective of the spacing between the capping beams or the existence of decking. Still further, this drill floor package support structure should preferably be, despite the enormity of its size and weight, capable of assembly at sea and should not require the jack-up rig to be towed into port for installation and preparation. The current invention is intended to satisfy these needs and to provide further related advantages.